I always thought of all this native fad to be temporary. Can't we expect with browsers to mature and device performance to improve, to have a platform that's more than good enough to build mobile apps in the future? Anyways, I'm hoping that the crowd regains some common sense and starts advocating the web over these damn fenced gardens.
Not really a start-up, but some things I've been working on, mostly in an attempt to get in the habit of shipping stuff opposed to writing.
1. Kill long meetings (http://killlongmeetings.com/). Counts the time and calculates the money you're spending in a meeting.
2. Where to pee in Antwerp (http://antwerp-wheretopee.apphb.com/). The city of Antwerp (Belgium) has opened up some of its data. This mobile web app get all the toilets in Antwerp and sorts them by distance. Shows that the browser is already capable of running these sort of apps; beats building 3 native apps.